The present invention relates to the use of an austenitic Ni--Cr--Mo--Nb--Fe-based alloy as a construction material for the manufacture of composite tubes with multiple coatings that satisfies demands in regard of high resistance towards chloride-induced and sulphide-induced corrosion, erosion and melt deposits, and that also was good resistance in reducing environments when used as evaporator tubes or superheater tubes in waste incinerators.
Such an external material component has been found advantageous if made out of a tube hollow that is made on the basis of the so-called Osprey-process which has been subjected to co-extrusion to a final tube where the inner tube component is a carbon steel or a low-alloy Cr--Mo-steel or any other approved standard boiler steel such as SS2352 (18.3 Cr, 10.1 Ni).
Composite tube means a tube consisting of two layer components which have a metallurgical bonding between themselves. Osprey-process means a process for tube fabrication wherein finely distributed drops of the metal is spray deposited on to the inner component in melt condition but which solidifies shortly afterwards. The composite tube is then fabricated by joint extrusion.
Evaporator tubes are those tubes in the incinerator where feeding water is heated by the heat from the combustion gases and which are being evaporated to the temperature that is given by the pressure in the tube.
Super-heater tubes are those tubes in the boiler where the steam is subject of overheating from the heat that comes from the combustion gases.
The tube solutions that today are primarily used for evaporator and super-heater tubes in waste incinerators are non-protected, or are tubing of carbon steel or low-alloy Cr--Mo-steel covered by stamp mass. Other types of protection means are blocks or shields made of ceramic or metallic material. Alternatively, those tubes could be covered by metal atomized layers or weld overlays on the tubes with more highly alloyed material with higher resistance.
The disadvantages observed with these alternatives are too short life times. Stamp masses come apart and cause free exposure of the tubes to gas attacks, whereby the ceramic or metallic shields get insufficient cooling and are rapidly subjected to corrosion and demand frequent maintenance. The weld overlays are being mixed up with the tube material and thereby are impairing corrosion properties. The iron content increases some 10% which promotes damaging iron chlorides to be formed in those tubes. Porosity, cracks or insufficient bonding at the interface between the weld overlay and the tube of carbon steel or low-alloy Cr--Mo-steel causes a risk that on welded/metallized layers will be subject of corrosion or come apart and thereby expose the underlying material for corrosion.